Substance use disorders are among the most common mental disorders, with approximately 9% of the population experiencing a substance use disorder in a given year. Moreover, it is estimated that substance use disorders cost 559 billion dollars a year in the United States (of which 181 billion is due to illicit substances) dueto lost productivity and costs of health care, crime, and drug enforcement. Unfortunately, the psychological mechanisms that maintain problematic substance use are still poorly understood, as evidenced by poor treatment outcomes for substance use (e.g., high treatment drop-out and high rates of relapse after treatment). Previous theory and research has established a link between the experience of negative affect (NA) and substance use for some substances (e.g., alcohol and nicotine). Yet, this relation has not been explored in illicit substances (e.g., cocain, heroin). Another factor that has been linked to substance use is self-control, with individuals wit poorer self-control being more likely to have problematic levels of substance use. To date, few studies have examined how self-control might moderate the relation between NA and substance use longitudinally. Further, no study has examined the interaction between self-control and NA in the prediction of illicit substance use. I predict that similar to alcohol and nicotine, recent A will predict future illicit substance use, which will be established by Aim 1. Furthermore, I predit that different aspects of self-control will moderate this relation (Aim 2). More specifically, I predict that the ability to detect the need for self-control will not moderate the relation between NA and illicit substance use. This is because identifying the need for self-control is necessary, but not sufficient, for successful self-control. Following this logic, it is predicted that two metods of self-control, adjusting behavior and persisting through temptation, will moderate the NA - illict substance relation, such that individuals with a higher ability to exert self-control will have a weaker relation between NA and illicit substance use compared to individuals with a lower ability to exert self-control. These relations will be established in a sample of individuals with substance use disorders who complete precise laboratory measures assessing different aspects of self-control. Subsequently, the participants will report weekly NA and substance use for four weeks. The results of this study will expand our knowledge of the maintenance factors of illicit substance use. This understanding, in turn, may be used to aid the development of more effective treatments for substance use disorders.